The present invention relates to a door lock for mounting in a mortise formed in a door jamb or door frame. More specifically, the present invention relates to a pivoting lock bolt for use with a jamb mounted lock. Jamb mounted locks of this kind are typically used to secure doors such as those found in penal institutions, as well as in other applications.
Locks that are designed to mount in door jambs typically utilize a lock bolt that pivots about a shaft and rotates back into its housing as the door is closed. This type of a lock bolt is typically called a "swing bolt." As a result of this pivoting action swing bolts require less mounting depth than standard "linear" bolts, which traverse straight back into their housings against a compression spring. Linear bolts typically require up to six inches of mounting depth to provide the bolt with enough throw to secure the door. To obtain this much throw linear bolts are typically mounted in the door itself. The strike plate and the receiving hole are located in the door jamb. This provides the necessary linear space to obtain the needed throw. Such linear bolts are typically used in residential and commercial applications.
However, in many applications it is desirable to mount the lock in the jamb rather than the door. This is especially true in penal institutions because the doors in these facilities are remotely actuated. Since remote actuation requires wires, conduit, and sometimes pneumatic tubes, if the lock were mounted in the door wires and conduit would be exposed when the door was open. This would present an unacceptable security risk. Moreover, use of a linear bolt in these applications requires a great deal of jamb depth to obtain enough throw on the bolt to adequately secure the door. Since the width of the door is fixed, this necessitates enlargement of the opening in the masonry wall to accommodate the wider jamb. In a penal institution, where the walls are generally concrete and the jambs are metal, such an enlargement adds significantly to the cost. Linear bolts are therefore often unacceptable in such applications.
On the other hand, because swing bolts rotate back into their housings by pivoting about a shaft, they require relatively little mounting depth compared to linear bolts. Thus, a swing bolt type lock may be mounted in a relatively small mortise formed in a narrow door jamb; the strike plate and the bolt receiving hole are located in the door. This eliminates the need for special frame preparation or for having an exceptionally large opening in the masonry wall to accommodate an exceptionally deep frame or jamb. Thus, jamb mounted locks utilizing swing bolts may be used in applications where there are constraints on the amount of space available, and also where it is economically advantageous to minimize the amount of building materials used in constructing the door frame.
In addition to the desirably shallow jamb mounting depth that swing bolts allow, another desireable feature in such a locking mechanism is that it have a narrow cross-sectional width. By reducing the width of the lock housing the width of the jamb or frame may be reduced accordingly since the mortise in which the lock mechanism is mounted may be relatively narrow. This reduces material costs and saves space. However, the width of the lock housing is limited somewhat by the relatively large width required for the swing bolt itself. With a swing bolt the angle of incidence between the door edge and the face of the swing bolt that the door edge strikes is critical. The angle of incidence must be steep enough to allow the force of the closing door to drive the swing bolt back into its housing.
Furthermore, in applications such as penal institutions it is especially necessary for the bolt to protrude as much as possible from the jamb to properly engage the receiving plate on the door. The further that the bolt protrudes from the jamb, the further the bolt will engage the receiving hole in the door, and the engagement of the door will be made more secure. It also increases tamper resistance. The need for as much protruding length as possible, when combined with the constraints on the width of the swing bolt, may result in a bolt which has too flat a bevel angle on the face of the bolt, relative to the door, to be driven back into the lock housing by the door when it is closed. The shape of the swing bolt is therefore critical to its efficient operation.
One solution to this problem has been to increase the cross-sectional depth of the bolt in order to increase the bevel angle on the face of the bolt relative to the door. This increases the angle of incidence between the door and the back of the bolt, and allows the door to drive the bolt back into its housing as the door is closed. But this solution also increases the width of the lock body and therefore the required dimensions of the door jamb. This increases the cost of manufacturing the lock and of the materials used in the door and frame.
Another solution to the problem of the bevel angle being too flat is to decrease the distance that the bolt protrudes from the lock body. This allows for a steeper bevel angle on the face of the bolt, allowing it to be driven back into the lock body as the door is closed. However, this solution is not satisfactory because assembly tolerances between the door and jamb, known in the industry as the reveal, allows enough variation that a swing bolt with a relatively short protrusion may not engage the receiving plate far enough to hold the door closed securely. This is especially important in penal institutions where maximum security and tamper resistance is desired. In addition, in the event of a fire the reveal may be increased due to warping of the door caused by the intense heat. If the bolt has a relatively short throw such that it engages the door only a short distance, the warping caused by the heat may be enough to cause the door to open inadvertently. Accordingly, a swing bolt assembly with a short protrusion is not satisfactory.
Another solution has been to provide a swing bolt that has a number of tapered camming surfaces to facilitate the forcing of the bolt back into its housing as the door is closed. Such a lock bolt is illustrated in U.S. Pat. No. 4,237,711 to Kambic. However, in that bolt the door first contacts the ridge line formed at the intersection of two camming surfaces. As the bolt is forced back into its housing the door traverses the ridge line until it ultimately contacts across the length of the camming surface nearest the pivot point. The door initially contacts only a small point on the ridge line. The pressure exerted on that point is very high and it is difficult therefore to initiate the rotating motion of the bolt. Once this motion has started the frictional forces drop until the door contacts across the length of the camming surface. If the bevel angle of the camming surface is too low relative to the face of the door, the door may stop.
Thus, there remains a need for a swing bolt which allows enough protrusion to sufficiently engage the receiving plate on the door to hold the door closed, is narrow in width, and which has a steeply angled bevel on the striking face so that it functions efficiently. The angle of incidence between the face of the bolt and the door must be sufficiently steep to allow the door to force the bolt back into its housing as the door closes.